1. Field of the Invention
This invention relates to control devices for a motor.
2. Description of the Related Art
The prior known bipolar drive method is exemplified in FIG. 5 where only the circuit for driving one coil L1 of the stepping motor is shown. An input signal DIR for changing over the direction of a current flowing to the coil is supplied to buffers G51 and G54 and inverters G52 and G53. The outputs of the buffers and inverters G51-G54 are supplied through resistors R51-R54 to the bases of transistors Tr51-Tr54 respectively. The transistors Tr51 and Tr52 are of NPN type, their emitters being grounded and their collectors being connected to the respective ends of the coil L.sub.1. The transistors Tr53 and Tr54 are of PNP type, their emitters being connected to an electric power source Vcc and their collectors being connected to the respective ends of the coil L.sub.1. (The transistor Tr54 is connected to the collector side of the transistor Tr51 and the transistor Tr53 is connected to the collector side of the transistor Tr52.)
Now assuming that the level of the signal DIR is high, then the transistors Tr51 and Tr53 are biased by the buffer and inverter G51 and G53, being in a conducting state. Suppose, at this time, the signal DIR has changed to low level, then the transistors Tr52 and Tr54 are biased to come into a conducting state by the inverter and buffer G52 and G54, while the transistors Tr51 and Tr53 simultaneously come into a non-conducting state. Here, no problem will arise if the conduction of the transistors Tr52 and Tr54 starts after the transistors Tr51 and Tr53 have completely come into a non-conducting state. In fact, it is before the transition of either or both of the transistors Tr51 and Tr53 to a non-conducting state is completed that the transistors Tr52 and Tr54 come into a conducting state. Therefore, it happens, though for a moment, that a through-current flows in a route: the power source Vcc.fwdarw.transistor Tr54.fwdarw.the transistor Tr51.fwdarw.the ground, or the power source Vcc.fwdarw.the transistor Tr53.fwdarw.the transistor Tr52.fwdarw.the ground.
Thus, the conventional type has a drawback that, when the direction of a driving current supplied to the stepping motor is changed over, an unnecessary current is allowed to flow to the driver, so that heat is generated therein and an unnecessary surplus of the load is given to the electric power source.